1. Field of the Invention
The present invention relates to an optical fiber which can be applied to a long-haul large-capacity optical communication system and, more particularly, to a dispersion-shifted fiber which is suitable for wavelength division multiplexing (WDM) communication and whose zero-dispersion wavelength is set within a desired range.
2. Related Background Art
A conventional optical communication system to which a single-mode optical fiber is applied as a transmission line often uses light in a 1.3- or 1.55-xcexcm wavelength band as communication signal light. Recently, however, use of light in a 1.55-xcexcm wavelength band increases from the viewpoint of a reduction in transmission loss in the transmission line. In particular, for a single-mode optical fiber (to be referred to as a 1.55-xcexcm single-mode optical fiber hereinafter) applied to a transmission line for light in the 1.55-xcexcm wavelength band, since the transmission loss in a silica-based single-mode optical fiber is minimized for light in the 1.55-xcexcm wavelength band, the wavelength dispersion (phenomenon that a pulse wave spreads due to the light propagation speed difference depending on wavelengths) is also designed to be zero for light in the 1.55-xcexcm wavelength band. Such a 1.55-xcexcm single-mode optical fiber whose zero-dispersion wavelength is shifted near the 1.55-xcexcm wavelength band is generally called a dispersion-shifted fiber.
As a conventional dispersion-shifted fiber, the sectional structure and composition of a dispersion-shifted fiber whose zero-dispersion wavelength is shifted near 1.55 xcexcm, and a method of manufacturing the same are disclosed in, e.g., Japanese Patent No. 2533083 (first prior art). The dispersion-shifted fiber of the first prior art has an inner core made of GeO2xe2x80x94SiO2 (SiO2 containing germanium dioxide), an outer core made of SiO2, and a cladding made of Fxe2x80x94SiO2 (SiO2 containing fluorine). The refractive index profile of the dispersion-shifted fiber of the first prior art is a so-called matched type profile which has no depressed part in a portion corresponding to the cladding. In this specification, an optical fiber having this matched type profile will be referred to as a matched fiber. On the other hand, a refractive index profile having a depressed part in a portion corresponding to the cladding is called a depressed cladding type profile. In this specification, an optical fiber having this depressed cladding type profile will be particularly referred to as a depressed fiber. In the structure of the dispersion-shifted fiber of the first prior art, only setting of the zero-dispersion wavelength near 1.55 xcexcm can be realized.
Japanese Patent Laid-Open No. 63-208005 (second prior art) discloses a dispersion-shifted fiber having a depressed cladding type profile, which has, around a core, a first cladding having a refractive index lower than that of the core, and, around the first cladding, a second cladding having a refractive index higher than that of the first cladding. The object of the dispersion-shifted fiber of the second prior art is to suppress wavelength dispersion over a wide wavelength band of 1.3 to 1.5 xcexcm.
In recent years, extensive studies of construction of high-speed large-capacity transmission systems have been made, and particularly, transmission systems employing wavelength division multiplexing (WDM) have received a great deal of attention. In this scheme, a plurality of signal light components having different wavelengths are simultaneously transmitted through one transmission line, so the quantity of data which can be transmitted largely increases.
To realize such a transmission system, various new specifications are required of an optical fiber to be used as a transmission line. The above-mentioned conventional dispersion-shifted fiber cannot cope with the requirements anymore.
The present inventors have examined the conventional dispersion-shifted fiber and found the following problems. The mode field diameter (MFD) of the conventional dispersion-shifted fiber is about 8 xcexcm. When the power of signal light increases, the influence of nonlinear optical effects tends to be generated. In addition, a variation in wavelength dispersion among various dispersion-shifted fibers applied to a transmission system is large. For this reason, when the signal light wavelength matches the zero-dispersion wavelength of the optical fiber, noise tends to be generated due to four-wave mixing as a nonlinear optical effect.
The nonlinear optical effect is a phenomenon that a signal light pulse is distorted as, e.g., the density of light intensity increases, and this is a major factor for restricting the transmission rate.
When, e.g., fluorine is added to adjust the refractive index of silica glass as a major component of the optical fiber, bubbles may be formed in the preform, or the preform itself may deform in sintering (making the preform transparent) the porous preform of the optical fiber. Flaws formed on the transparent glass surface (preform surface) due to the added impurity may break the optical fiber at the time of drawing.
The present invention has been made to solve the above problems, and has as its object to provide an optical fiber having a large MFD and a structure for effectively suppressing the influence of nonlinear optical effects, and a method of manufacturing the same which effectively prevents bubble occurrence in a transparent preform, deformation of the preform, and flaws on the preform surface during the manufacture of the optical fiber (drawing process).
An optical fiber according to the present invention is a dispersion-shifted fiber whose MFD is 8.6 xcexcm or more, and preferably, 9 xcexcm or more, and whose zero-dispersion wavelength is shifted to the long or short wavelength side of 1.55 xcexcm, i.e., a typical signal light wavelength. The optical fiber is a single-mode optical fiber mainly containing silica glass. In this dispersion-shifted fiber, the zero-dispersion wavelength is shifted from the signal light wavelength by a predetermined amount to intentionally generate wavelength dispersion and suppress the influence of nonlinear optical effects. Therefore, a transmission system which allows variations in zero-dispersion wavelength among dispersion-shifted fibers can be constructed.
According to the first embodiment according to the present invention, there is provided an optical fiber comprising a first core 10 (inner core) having a first refractive index n1, a second core 20 (outer core) provided around an outer periphery of the inner core 10 and having a second refractive index n2 lower than the first refractive index n1, a first cladding 30 (inner cladding) provided around an outer periphery of the outer core 20 and having a third refractive index n3 lower than the second refractive index n2, and a second cladding 40 (outer cladding) provided around an outer periphery of the inner cladding 30 and having a fourth refractive index n4 higher than the third refractive index n3 and lower than the second refractive index n2, as shown in FIG. 1.
In particular, an optical fiber 1 according to the first embodiment has a depressed cladding type profile, as is apparent from the above-described structure. The outer core 20 has an outer diameter of 25 to 40 xcexcm.
This depressed cladding type profile can be realized when the following basic composition is employed: the inner core 10 is made of silica glass containing at least germanium dioxide as an index increaser (GeO2xe2x80x94SiO2); the outer core 20, silica glass essentially containing no germanium dioxide (SiO2) or silica glass containing germanium dioxide (GeO2xe2x80x94SiO2); the inner cladding 30, silica glass containing at least fluorine (index reducer) (Fxe2x80x94SiO2); and the outer cladding 40, silica glass containing at least fluorine (Fxe2x80x94SiO2). When the sectional area (plane perpendicular to the signal light propagation direction) of the outer core 20 is increased, as in this optical fiber (the outer diameter is 25 to 40 xcexcm), and GeO2 is doped in the outer core 20, the refractive index profile can hardly be flattened in the radial direction of the outer core 20. For this reason, the outer core 20 preferably contains no germanium dioxide.
According to the first embodiment according to the present invention, there is provided a method of manufacturing the optical fiber 1, comprising the first step of forming, by vapor phase synthesis, a porous preform (soot preform) whose central portion along a longitudinal direction corresponds to the inner core 10 and whose peripheral portion around the central portion corresponds to the outer core 20, the second step of sintering the porous preform to obtain a core glass preform, the third step of elongating the core glass preform obtained in the second step to a desired outer diameter and depositing a first porous glass body (soot body) corresponding to the inner cladding 30 on an outer surface of the elongated core glass preform by vapor phase synthesis to obtain a first composite preform, the fourth step of sintering the first composite preform obtained in the third step in an atmosphere containing a fluorine raw material to obtain an intermediate preform, the fifth step of elongating the intermediate preform obtained in the fourth step to a desired outer diameter and depositing a second porous glass body (soot body) corresponding to the outer cladding 40 on an outer surface of the elongated intermediate preform by vapor phase synthesis to obtain a second composite preform, the sixth step of sintering the second composite preform obtained in the fifth step to obtain an optical fiber preform, and the seventh step of drawing the optical fiber preform obtained in the sixth step while heating one end of the optical fiber preform. Sintering of the second composite preform in the sixth step is performed in an atmosphere containing a fluorine raw material.
In the method of manufacturing the optical fiber of the first embodiment, since the outer diameter of the outer core 20 of the optical fiber 1 to be manufactured is as large as 25 to 40 xcexcm, glass synthesis (third step) of the first porous glass body as the inner cladding 30 and glass synthesis (fifth step) of the second porous glass body as the outer cladding 40 are performed by vapor phase synthesis such as VAD (Vapor phase Axial Deposition) or OVD (Outside Vapor phase Deposition). If portions corresponding to the inner and outer claddings 30 and 40 cannot be manufactured by vapor phase synthesis, they are manufactured by rod-in-collapse method. In this case, the size of a resultant preform is limited, so the productivity is difficult to increase.
In the method of manufacturing the optical fiber 1 of the first embodiment, the outer diameter of the outer core 20 of the optical fiber to be manufactured is set to be 25 to 40 xcexcm, so the third step can be performed using vapor phase synthesis (the fifth step can also be performed by vapor phase synthesis). Each of the above-described steps is adjusted such that the outer diameter of the outer core 20 of the optical fiber 1 falls within the desired range.
In the method of manufacturing the optical fiber 1 of the first embodiment, the porous preform obtained in the first step is heated in an atmosphere containing a halogen gas before the second step to dehydrate the porous preform. Therefore, the inner and outer cores 10 and 20 of the resultant optical fiber contain chlorine at a predetermined concentration. The first composite preform obtained in the third step is also heated in an atmosphere containing a halogen gas before the fourth step to dehydrate the first porous glass body. Therefore, the inner cladding 30 of the resultant optical fiber also has chlorine at a predetermined concentration. A preform region corresponding to the inner cladding 30 is subjected to dehydration because even when the porous glass body (soot body) is formed on the outer surface of the outer core 20 by vapor phase synthesis (method of depositing fine glass particles using flame hydrolytic reaction), the influence of OH absorption in the resultant optical fiber can be relaxed.
The chlorine content in the inner cladding 30 is preferably lower than that in the inner and outer cores 10 and 20. Chlorine is known as a dopant for increasing the refractive index. When chlorine is doped in the core region (including the inner and outer cores 10 and 20), the contents of fluorine (index reducer) doped in the cladding region (including the inner and outer claddings 30 and 40) can be reduced without changing the refractive index profile of the optical fiber.
As described above, the contents of fluorine in the cladding region can be reduced. For this reason, instead of the sixth step of sintering the second composite preform in the atmosphere containing the fluorine raw material (including doping of fluorine), in the fifth step, the second porous glass body can be deposited by supplying a fluoride gas to the outer surface of the intermediate preform obtained in the fourth step to obtain the second composite preform. In this case, in the sixth step, only sintering of the second composite preform obtained in the fifth step is performed. For this reason, the sintering time can be shortened, and the productivity can be largely improved.
The above-described dehydration process, i.e., the heating process performed in the atmosphere containing the halogen gas may be performed for the second composite preform obtained in the fifth step before the sixth step. In this case, the outer cladding 40 of the resultant optical fiber always contains chlorine.
As the halogen gas used for dehydration, SiCl4 is preferably used. According to the second embodiment according to the present invention, there is provided an optical fiber comprising a first core 160 (inner core) having a first refractive index n1, a second core 170 (outer core) provided around an outer periphery of the inner core 160 and having a second refractive index n2 lower than the first refractive index n1, a first cladding 180 (inner cladding) provided around an outer periphery of the outer core 170 and having a third refractive index n3 lower than the second refractive index n2, and a second cladding 190 (outer cladding) provided around an outer periphery of the inner cladding 180 and having a fourth refractive index n4 higher than the third refractive index n3.
The refractive indices of the inner and outer claddings 180 and 190 of an optical fiber 150 increase in the radial direction from the inner side toward the outer side of each of the claddings 180 and 190. The inner and outer claddings 180 and 190 contain fluorine as a dopant for adjusting the refractive index.
The optical fiber of the second embodiment has a depressed cladding type profile. This refractive index profile can be realized when the following basic composition is employed: the inner core 160 is made of silica glass containing at least germanium dioxide as an index increaser (GeO2xe2x80x94SiO2); the outer core 170, silica glass essentially containing no germanium dioxide (SiO2) or silica glass containing germanium dioxide (GeO2xe2x80x94SiO2); the inner cladding 180, silica glass containing at least fluorine (index reducer) (Fxe2x80x94SiO2); and the outer cladding 190, silica glass containing at least fluorine (Fxe2x80x94SiO2). When GeO2 is doped in the outer core 170, the refractive index profile can hardly be flattened. For this reason, the outer core 20 preferably contains no germanium dioxide. In addition, the outer diameter of the outer core 170 is preferably 25 to 40 xcexcm to effectively prevent the influence of nonlinear optical effects, as in the above-described first embodiment.
According to the second embodiment according to the present invention, there is provided a method of manufacturing the optical fiber 150, comprising at least the first step (FIGS. 2 to 7) of forming, by vapor phase synthesis such as VAD (Vapor phase Axial Deposition) or OVD (Outside Vapor phase Deposition), a porous preform 50 whose central portion along a longitudinal direction corresponds to the inner core 160 and whose peripheral portion around the central portion corresponds to the outer core 170, and sintering the porous preform 50 to obtain a core glass preform 51, the second step of elongating the core glass preform 51 to a desired outer diameter (FIG. 8), heating, in an atmosphere containing a fluorine raw material having a predetermined concentration at a predetermined temperature, a first composite preform 52 (FIG. 9) obtained by depositing a first porous glass body corresponding to the inner cladding 180 on an outer surface of the elongated core glass preform 51 by vapor phase synthesis, and vitrifying the first composite preform 52 after the atmosphere temperature is increased and the concentration of the fluorine raw material contained in the atmosphere is changed, thereby obtaining a transparent intermediate preform 53 (FIG. 12), and the third step (FIG. 11) of elongating the intermediate preform 53 to a predetermined outer diameter (FIG. 8), heating, in an atmosphere containing a fluorine raw material having a predetermined concentration at a predetermined temperature, a second composite preform 54 (FIG. 9) obtained by depositing a second porous glass body corresponding to the outer cladding 190 on an outer surface of the elongated intermediate preform 53 by vapor phase synthesis, and making the second composite preform 54 transparent after the atmosphere temperature is increased and the concentration of the fluorine raw material contained in the atmosphere is changed (FIG. 12), thereby obtaining a transparent optical fiber preform 55.
In the second and third steps in the method of manufacturing the optical fiber 150 of the second embodiment, the supply amount of the fluorine raw material doped in the preform regions corresponding to the inner and outer claddings 180 and 190 as an index adjustment material is adjusted (the fluorine concentration in the atmosphere in the above-described heating and vitrifying processes is adjusted). More specifically, when fluorine as an index adjustment material is doped in the glass material, flaws and the like are readily formed on the glass surface. To prevent this, in the second step, the concentration of the fluorine raw material contained in the atmosphere when heating the first composite preform 52 is set to be higher than that in the atmosphere when making the first composite preform 52 transparent, and in the third step as well, the concentration of the fluorine raw material contained in the atmosphere when heating the second composite preform 54 is set to be higher than that in the atmosphere when making the second composite preform 54 transparent.
When the fluorine concentration in the preform reason corresponding to the inner cladding 100 is lowered in the radial direction from the inner region to the peripheral region (the refractive index of the preform region is increased in the radial direction), the index difference from the inner region of the preform region, which corresponds to the outer cladding 190, is made small. In other words, when the concentration of fluorine to be doped in a predetermined preform region is adjusted such that the fluorine concentration is lowered at the interface between the inner cladding 180 and the outer cladding 190 of the resultant optical fiber, bubble occurrence in the preform or deformation of the preform itself can be effectively prevented during sintering of the preform. When contents of fluorine in the preform region corresponding to the outer cladding 190 is also adjusted in the radial direction, deformation in the heating process or flaws on the outer surface of the preform or outer surface of the resultant optical fiber in handling during the manufacture can be effectively prevented.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.